Vertical profile of Chl-a concentration was measured on site with a fluorescent photometer at eleven stations in the Shichigashuku reservoir once a day during a period of phytoplankton bloom in late spring of 1996. The unsteady and unhomogeneous characteristics of the bloom was clarified as follows: (1) Asterionella, the dominant species, increases at the bottom of surface mixed layer where water column is gravitationally stable and temperature is around 15 °C which is suitable for them to breed and grow. Asterionella decays abruptly when the suitable layer moves down below compensation depth because of the deepening of thermocline at the beginning of summer. (2) Asterionella tends to concentrate in an area along the north boundary of the reservoir. This fact implies the existence of a mechanism for nutrient supply in the area.
A wind induced current under the thermal condition of late spring is numerically simulated by using 3-D k-ε model in order to investigate the mechanism of nutrient supply. The result shows that an upwelling flow takes place in the north area and transports materials from the bottom to the surface under the action of a northwest wind which is prevailing in the season of phytoplankton bloom.